Radiation therapy has been employed to treat tumorous tissue. In radiation therapy, a high energy beam is applied from an external source towards the patient. The external source, which may be rotating (as in the case for arc therapy), produces a collimated beam of radiation that is directed into the patient to the target site. The dose and placement of the dose must be accurately controlled to ensure that the tumor receives sufficient radiation, and that damage to the surrounding healthy tissue is minimized.
Sometimes, before a radiation therapy is performed, the target region of the patient is imaged using a CT system for diagnostic purpose, or for treatment planning. For the case in which the target region moves in a periodic motion (e.g., due to breathing), the CT system may be used to determine volumetric images of the target when the target is at different breathing states, so that the volumetric images may be played back as a video stream. One such imaging technique is known as 4D cone beam CT (CBCT). For such purpose, projection images of the target when the target is at different breathing states are acquired, and a breathing monitoring device is used to determine breathing states of the patient as the CT system acquires the projection images. After the imaging session, the projection images are then sorted according to the recorded breathing states of the patient when the corresponding projection images are acquired. The breathing monitoring device is required to track the breathing states accurately. The tracked breathing states cannot be too coarse (e.g., they cannot merely indicate whether the patient is at an inhale state or an exhale state) because otherwise, the resulting video stream would be too coarse for diagnostic and treatment planning purposes.
However, Applicant of the subject application has determined that 4D CBCT imaging is time consuming and the large number of projections may potentially expose the patient to excessive x-ray imaging does. Accordingly, instead of generating a video using a sequence of CT images, it may be desirable to generate the video using a sequence of digital tomosynthesis images (4D DTS).
Digital tomosynthesis image is an image that is reconstructed using projection images, wherein the number of projection images involved may be less than those for a CT image. However, existing techniques for obtaining projection images for reconstruction of a digital tomosynthesis image may not be desirable for 4D DTS. This is because existing imaging techniques are designed to perform rapid imaging in the interest of reducing the duration of an imaging session. However, rapid imaging may result in projections within each phase bin (i.e., the projection images that are used to reconstruct the tomosynthesis image for a certain phase or phase range) having non-uniform angular distribution, which may in turn degrades the geometric fidelity and depth resolution of the resulting digital tomosynthesis image.